1. Field of the Invention
The present invention relates to a method utilized in a wireless communication and communication device thereof, and more particularly, to a method of handling TTI bundling operation improving transmission in a wireless communication system and communication device thereof.
2. Description of the Prior Art
A long-term evolution (LTE) system, initiated by the third generation partnership project (3GPP), is now being regarded as a new radio interface and radio network architecture that provides a high data rate, low latency, packet optimization, and improved system capacity and coverage. In the LTE system, an evolved universal terrestrial radio access network (E-UTRAN) includes a plurality of evolved Node-Bs (eNBs) and communicates with a plurality of mobile stations, also referred as user equipments (UEs).
A radio interface protocol of the LTE system includes three layers: the Physical Layer (L1), the Data Link Layer (L2), and the Network Layer (L3), wherein a control plane of L3 is a Radio Resource Control (RRC) layer, and L2 is further divided into a Packet Data Convergence Protocol (PDCP) layer, a Radio Link Control (RLC) layer and a Medium Access Control (MAC) layer.
The main services and functions of the MAC layer include mapping between logical channels and transport channels; multiplexing/demultiplexing of RLC PDUs belonging to one or different radio bearers into/from transport blocks (TB) delivered to/from the physical layer on transport channels; buffer status reporting; power headroom reporting; error correction through HARQ (Hybrid Automatic Repeat Request); priority handling between logical channels of one UE; priority handling between UEs by means of dynamic scheduling; TTI (Transmission Time Interval) bundling transmission and padding.
TTI bundling transmission is introduced to improve LTE uplink coverage without the issues of overhead associated with L2 segmentation and ACK (Acknowledgement)/NACK (Negative Acknowledgement) errors. The UEs in cell boundary can reduce transmission delay by means of TTI bundling transmission. The activation and deactivation of TTI bundling transmission is controlled by RRC signaling message, e.g. RRC Connection Reconfiguration message.
In the MAC layer, adaptive and non-adaptive retransmissions are used. The adaptive retransmissions is performed on resources with a MCS (Modulation and Coding Scheme) indicated on a PDCCH (Physical Downlink Control Channel). The non-adaptive retransmission is performed on the same resource and with the same MCS as was used for the last made transmission attempt.
If TTI bundling is configured by the RRC layer, a parameter TTI_BUNDLE_SIZE provides the number of TTIs of a TTI bundle. Within a TTI bundle, HARQ retransmissions are non-adaptive and shall be performed without waiting for feedbacks (e.g. NACK or ACK) related to previous transmissions according to the parameter TTI_BUNDLE_SIZE. A feedback for a TTI bundle is only received for a specific TTI corresponding to TTI_BUNDLE_SIZE. A retransmission of a TTI bundle is also a TTI bundle. For transmission of an uplink message containing a C-RNTI (Cell Radio Network Temporary Identifier) MAC control element or an uplink message including a CCCH (Common Control Channel) SDU (Service Data Unit) during a random access procedure, the TTI bundling does not apply.
Activation and deactivation of TTI bundling transmission are configured by RRC signaling message. However, TTI bundling transmission is done in MAC/PHY layers. Therefore, the activation or deactivation of the TTI bundle may not be synchronized between the UE and the eNB. When the UE and the eNB activate or deactivate the TTI bundling operation in the different time, transmission interference or degradation of the system performance can occur.
Take an issue 1 for example. Please refer to FIG. 1, which is a schematic diagram of a TTI bundling operation according to the prior art. The UE activates the TTI bundling operation with a HARQ process id (identity) 0 at TTI 1. The TTI_BUNDLE_SIZE is set to 4. In this situation, transport blacks of the TTI bundle are transmitted from TTI 1 to TTI 4. When activation or deactivation of the TTI bundling operation is not synchronized between the UE and the eNB, the UE receives the feedback of the TTI bundle in the wrong TTI as being shown in FIG. 1 where an HARQ NACK is received in TTI 5. On the standpoint of the UE, the UE does not know if the received feedback corresponds to the TTI bundle or not.
The prior art does not specify when the UE shall perform a retransmission for a TTI bundling transmission after the feedback of the TTI bundling transmission is received. According to the prior art, a retransmission of a non-TTI-bundling transmission is performed in a TTI(n+4) when a feedback of the non TTI bundling transmission is received in a TTI(n). If a non-adaptive retransmission of the TTI bundle in FIG. 2 is performed based on the foregoing specification clause and thereby performed in TTI 9, the system performance may be downgraded since the non-adaptive retransmission is unnecessary (if the HARQ NACK does not correspond to the TTI bundle) or causes interference to transmissions of other UEs. For example, if a UE1 performs the non-adaptive retransmission in TTI 9 that has been allocated to a UE2 by the eNB due to the lack of synchronization of the TTI bundling activation, the transmission interference occurs.
Take an issue 2 for example. Please refer to FIG. 2, which is a schematic diagram of a TTI bundling operation according to the prior art. The TTI bundling operation is activated at the UE with a HARQ process id 0, and corresponding transport blocks are transmitted in a TTI bundle of TTIs 1-4. Due to the lack of synchronization of the TTI bundling activation, a HARQ ACK for the TTI bundle is received in TTI 5. No HARQ ACK is received in TTI 8 where the UE shall receive a feedback of the TTI bundle. As a result, a retransmission of the TTI bundle occurs in TTIs 17, 18, 19, and 20, configured by RRC signaling according to the prior art. This retransmission is unnecessary and wastes UE power since the eNB has feedback with the HARQ ACK for the TTI bundle. In addition, the retransmission may make interference to other UEs if any of the TTIs 17-20 has been allocated to other UEs.
In addition to the feedback, PDCCH information may be received during the period of waiting the feedback of a TTI bundle. The prior art does not specify how the UE shall deal with this situation, and thereby can cause an unnecessary retransmission or transmission interference.
Take an issue 3 for example. Please refer to FIG. 3, which is a schematic diagram of a TTI bundling operation according to the prior art. The TTI bundling operation is activated at the UE with a HARQ process id 0, and corresponding transport blocks are transmitted in a TTI bundle of TTIs 1-4. PDCCH information carrying an uplink (UL) grant including a MCS for an adaptive retransmission is received in TTI 5. This means that the eNB indicates the UE to perform an uplink transmission with the MCS in the adaptive way. If the adaptive retransmission in TTI 9 (5+4) occurs, the system performance is downgraded due to the unnecessary retransmission or transmission interference to other UEs.
During TTI bundling transmission, the UE possibly receives a request for deactivating the TTI bundling operation, also causing an unnecessary retransmission or transmission interference.
Take an issue 4 for example. Please refer to FIG. 4, which is a schematic diagram of a TTI bundling operation according to the prior art. The TTI bundling operation is activated at the UE with a HARQ process id 0, and corresponding transport blocks are transmitted in a TTI bundle of TTIs 1-4. The UE does not receive a HARQ NACK in TTI 8, and thereby a retransmission is configured to be performed in a TTI bundle of TTIs 17-20. The TTI bundling is deactivated at the UE with a HARQ process id 0 in TTI 18. According to the prior art, retransmissions in TTI 19 and 20 are cancelled. However, the prior art does not clearly specify when the UE receives a HARQ feedback of the retransmission. If the HARQ feedback of the retransmission is a NACK, the 2rd retransmission shall be performed. Even according to the retransmission clause for the non TTI bundling operation, the prior art does not specify that the UE shall perform the retransmission in TTI 22 or TTI 24. The 2rd retransmission is possibly performed at a TTI unexpected by the eNB, and thus an unnecessary retransmission or transmission interference to other UEs occurs, thereby downgrading the system performance.
Take an issue 5 for example. In the prior art, the TTI bundling operation does not apply for transmission of the uplink message containing the C-RNTI MAC control element or the uplink message including the CCCH SDU during the random access procedure (contention-based random access). On the other hand, the TTI bundling operation is applied for an MAC PDU transmission granted by a random access response which is triggered by a PDCCH order with a dedicated preamble (non-contention-based random access). Since the MAC PDU granted by the random access response contains no uplink data, using the TTI bundle to transmit the MAC PDU makes unnecessary retransmissions and wastes UE power.